Mechanism of the idling-turnover reaction of the large (Klenow) fragment of Escherichia coli DNA polymerase I.

Abstract
The mechanism of the idling-turnover reaction catalyzed by the large (Klenow) fragment of Escherichia coli DNA polymerase I has been investigated. The reaction cycle involved is one of excision/incorporation, in which the 3'' deoxynucleotide residue of the primer DNA strand is partitioned into its 5''-mono and 5''-triphosphate derivaties, respectively. Mechanistic studies suggest the 5''-monophosphate product is formed in the first step by simple 3'' .fwdarw. 5'' exonucleolytic cleavage. Rapid polymerization follows with the concomitant release of inorganic pyrophosphate. In the second step, the 5''-triphosphate product is generated by a pyrophosphorolysis reaction, which, despite the low concentration of pyrophosphate that has accumulated, occurs at a rate that is comparable with that of the parallel 3'' .fwdarw. 5'' hydrolysis reaction.

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